The present invention relates to an improved multi-stage process for producing multi-modal, preferably bimodal linear low density polyethylene (LLDPE) in the presence of an improved solid vanadium-containing Ziegler-Natta catalyst system, to LLDPE compositions obtainable by this process showing improved comonomer composition distribution and to articles manufactured therefrom.
One of the main challenges when producing linear low density polyethylene (LLDPE) grades in full scale is the formation of chunks, lumps and sheeting. These problems are very pronounced when using a conventional Ziegler-Natta type of catalyst. One of the main reasons is the poor comonomer composition distribution of such catalysts, i.e. the problem in copolymerizing ethylene with C3- to C10-alpha-olefins in the presence of conventional types of Ziegler-Natta catalyst compositions is, that the comonomers tend to be irregularly distributed over the molecule chains yielding copolymers with uneven or poor comonomer composition distribution (CCD), which can be detected for example by TREF (temperature rising elution fractionation) method, Differential Scanning calorimetry (DSC), GPC-FTIR (Gel Permeation Chromatography coupled with Fourier transform infrared spectroscopy instruments) or measuring the amount of xylene soluble polymer fraction.
A further problem that is often encountered with the prior art catalysts is that it is difficult to produce an ethylene copolymer having a very high molecular weight and a low density. Especially this problem is apparent when producing a bimodal ethylene copolymer, where, for instance, in a first stage a low molecular weight copolymer component is produced in the presence of a high concentration of hydrogen. The polymer produced in the first stage is then directed to a second polymerization stage to produce the high molecular weight copolymer component, in the presence of the low molecular weight copolymer component. At this stage the concentration of hydrogen needs to be low. Unfortunately a small stream of hydrogen is carried over from the first polymerization stage to the second polymerization stage. If the catalyst is very sensitive to hydrogen, like some single site catalysts, then the molecular weight of the polymer component produced in the second stage is reduced by the hydrogen carried over from the first polymerization stage. This makes it impossible to produce bimodal ethylene copolymers having a high weight average molecular weight and a low density with good CCD.
As is also known in the art, conventional ZN catalysts tend to produce ethylene polymers having broad molecular weight distributions.
The broadness of a MWD may be described by the ratio of the weight average molecular weight (Mw) and the number average molecular weight (Mn). A high value of Mw/Mn (MWD) indicates a broad molecular weight distribution.
Polyethylene requires for certain applications a bimodal distribution of molecular weight in order to yield optimal structural characteristics and physical properties. This can be achieved by ensuring that the polyethylene contains a component having a lower molecular weight (LMW component) and a component having a higher molecular weight (HMW component). The lower molecular weight mode imbues the final polymer with the desired processibility, whilst the higher molecular weight mode imbues the product with the necessary durability and hardness.
When conventional ZN catalysts are used to produce bimodal polymers both the lower and the higher molecular weight components tend to have a broad MWD. In particular the HMW components tend to have significant lower molecular weight “tails” which have deleterious affects on the mechanical properties of the polymer and on articles made from the polymer.
To solve these problems metallocene catalysts (single site catalysts) have been used, which have a very narrow molecular weight distribution, very controlled active site structure and copolymerize comonomers only into the higher molecular weight part, leading to very narrow composition distributions in slurry and gas phase conditions, minimizing the effect of improper low density low molecular weight polymers. These copolymers, of the same chemical composition, are endowed with superior properties as compared with traditional copolymers. Such single site catalyst systems, employing organometallic compounds and aluminoxane, can provide improved control of MWD and branching compositional distributions compared to traditional Ziegler-Natta catalyst systems.
However, the solubility of organometallic compounds and cocatalysts such as methylaluminoxane (MAO) requires immobilization processes on inorganic supports in systems that are costly. Accordingly, it can be difficult to apply single site catalysts in existing polymerization processes without major process modification and capital investments. So the application of such systems for producing LLDPE has its drawbacks. Also, if the catalyst is very sensitive to hydrogen, as many single site catalysts are, then, as stated above, the molecular weight of the polymer component produced in the second stage is reduced by the hydrogen carried over from the first polymerization stage. This makes it difficult to produce bimodal ethylene copolymers having a high weight average molecular weight and a low density with good CCD with single site catalysts.
Hence, there is still a need for improved processes which avoid the disadvantages and lacks in well-known multi-stage processes using conventional Ziegler-Natta (ZN) and single site (SS) catalyst systems and provide bimodal LLDPE having a narrow molecular weight distribution and improved comonomer composition distribution and thus overcome known problems of ZN- and SS-catalyst systems.
Several multistage processes using different ZN-catalysts have been described in the literature.
For example WO 2006/014475 describes a process for producing an ethylene polymer composition in a multistage process, wherein in the first stage ethylene is polymerized alone or with a comonomer to produce ethylene polymer, transferring the polymer produced in the first stage into a second stage, in which ethylene is polymerized alone or with a comonomer, in the presence of the polymer produced in the first stage. The first stage is a slurry polymerization stage and the polymerization is carried out in the presence of a catalyst system comprising:                a) a solid catalyst precursor, comprising a transition metal selected from titanium and vanadium; magnesium; a halide; optionally an electron donor; and a solid particulate material comprising an inorganic oxide, wherein the median particle diameter of the solid precursor, D50, is from 1 to 13 micrometers; and        b) an organoaluminium compound.        
By using this catalyst system the gel levels in ethylene polymer compositions produced in a multistage process, wherein the first stage is carried out in a slurry reactor, should be reduced. Preferably the second stage is also a slurry polymerization.
The preferred catalyst precursor used according to WO 2006/014475 has the formula MgaTi(OR)bXc(ED)d wherein R is an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms or COR′, wherein R′ is an aliphatic or aromatic hydrocarbon radical having 1 to 14 carbon atoms; each OR group is the same or different; each X is independently Cl, Br or iodine; ED is an electron donor; a is 0.5 to 56; b is 0, 1 or 2; c is 2 to 116 and d is less than or equal to 1.5a+4.
However WO 2006/014475 does not disclose the possibility of narrowing the molecular weight distribution of a bimodal ethylene polymer and improving the comonomer composition distribution by using special ZN-catalysts.
WO 2007/051607 describes the use of a modified ZN-catalyst system for tailoring the properties of a multimodal ethylene polymer to influence the molecular weight distribution (MWD) of a higher molecular weight (HMW) component whilst essentially having no effect on the MWD of the lower molecular weight (LMW) component.
The multimodal (e.g. bimodal) ethylene polymers having a LMW component and a HMW component are produced by polymerizing ethylene and optionally at least one further alpha olefin in at least two stages, wherein at least one stage is carried out in a slurry in the presence of a Ziegler-Natta catalyst comprising an electron donor that is an ether. The resulting ethylene polymer produced according to WO 2007/051607 in a full scale two
stage polymerization process preferably has a MFR2 of 10 to 1000 g/10 min measured according to ISO 1133 at 190° C. and under 2.16 kg load and can be used for the manufacture of films and pipe.
None of these literatures suggests the possibility of using a vanadium-containing Ziegler-Natta catalyst system in a multistage process which enables to improve the CCD, to narrow the MWD and which is also active in the presence of hydrogen, allowing the desired HMW component (lower MFR2) to be achieved in the later polymerization step. Furthermore there is no disclosure to tailor the placement of comonomer into the high molecular weight fraction of the polymer and also to tailor the molecular weight profile of the high molecular weight fraction of the polymer.
Therefore there is still a need for a process which provides multi-, preferably bimodal linear low density polyethylene with controllable molecular weight distribution as well as comonomer composition distribution even when hydrogen is present.
In particular a process which produces multi-, preferably bimodal polymers having a narrow MWD, improved CCD and a higher molecular weight component without a significant lower molecular weight tail, but presence of a high molecular weight tail is desired.